First I have heard of it. I gather Parler benefited by hosting these groups?...Was it really that bad that all the vendors had to cancel Parler the social media site, to the point that it was forced to shut down..?
If someone's planning something stupid like trying to overthrow the US capital they can get a forum up (URL, hosting and all) for less than 25$ a month. The software is free. You can also get a Discord channel for free. You don't need to use somebody else's site. Parler chose to support whatever so that it could get the traffic.
I'm not saying you can start a 'Good' forum that way, but good enough for government work.